TM 5-822-5/AFM 88-7, Chap. 1
materials with fines within the stated limits.
base, and subbase to limit the penetration of frost
Occasionally, GC or SC materials may occur within
into the frost-susceptible subgrade to an acceptable
the F. group, although they will normally fall into
amount. Included also in this method is a design
the F3 category. The basis for division be- tween
approach which determines the thickness of pave-
the Fl and F. groups is that Fl materials may he
ment, base, and subbase necessary to prevent the
expected to show higher hearing capacity than F.
penetration of frost into the subgrade. Prevention of
materials during thaw, even though both may have
frost penetration into the subgrade is nearly always
experienced equal ice segregation.
uneconomical and unnecessary, and will not he used
c. Varved clays. Valved clays consisting of alter-
to design pavements to serve conventional traffic,
nating layers of silts and clays are likely to combine
except when approved by HQUSACE (CEMP-ET)
the undesirable properties of both silts and clays.
or the appropriate Air Force Major Command.
b. Reduced subgrade strength method. The
These and other stratified fine-grained sediments
may be hard to classify for frost design. Since such
second method does not seek to limit the penetra-
soils are likely to heave and soften more readily
tion of frost into the subgrade, but it determines the
than homogeneous soils with equal average water
thickness of pavement, base, and subbase that will
contents, the classification of the material of highest
adequately carry traffic loads over the design period
frost susceptibility should be adopted for design.
of years, each of which includes one or more
Usually, this will place the overall deposit in the F4
periods during which the subgrade supporting
category.
capacity is sharply reduced by frost melting. This
d. Special conditions. Under special conditions
approach relies on uniform subgrade conditions,
the frost group classification adopted for design
adequate subgrade preparation techniques, and
may be permitted to differ from that obtained by
transitions for adequate control of pavement
application of the above frost group definitions.
roughness resulting from differential frost heave.
This will, however, he subject to the specific ap-
proval of HQUSACE (CEMP-ET) or the appropri-
18-5. Selection of Design Method.
ate Air Force Major Command if the difference is
In most cases the choice of the pavement design
not greater than one frost group number and if
method will he made in favor of the one that gives
complete justification for the variation is presented.
the lower cost. Exceptions dictating the choice of
Such justification may take into account special
the limited subgrade frost penetration method, even
conditions of subgrade moisture or soil uniformity,
at higher cost, include pavements in locations where
in addition to soil gradation and plasticity, and
subgrade soils are so extremely variable (as, for
should include data on performance of existing
example, in some glaciated areas) that the required
pavements near those proposed to be constructed.
subgrade preparation techniques could not be
expected to provide sufficient protection against
18-4. Alternative Methods of Thickness Design.
differential frost heave. In other cases special
The thickness design process is the determination of
operational demands on the pavement might dictate
the required thickness for each layer of a pavement
unusually severe restrictions on tolerable pavement
system and of the combined thickness of all layers
roughness, requiring that subgrade frost penetration
above the subgrade. Its objective is determining the
he strictly limited or even prevented. If the use of
lowest-cost pavement system whose rate of
limited subgrade frost penetration method is not
deterioration under traffic loads and environmental
required, tentative designs must be prepared by
conditions will be acceptably low. In seasonal frost
both methods for comparison of costs. Also, a
areas the thickness design process must include the
tentative design must he prepared following the
effects of frost action. Two methods are prescribed
nonfrost-design criteria, since the thickness
for determining the thickness design of a pavement
requirements under nonfrost-criteria must be met in
that will have adequate resistance to distortion by
addition to the frost design requirements.
frost heave and cracking and distortion under traffic
loads as affected by seasonal variation of supporting
18-6. Limited Subgrade Frost Penetration.
capacity, including possible severe weakening
This method of design for seasonal frost conditions
during frost-melting periods.
should he used where it requires less thickness than
a. Limited subgrade frost penetration method.
the reduced subgrade strength method. Its use is
The first method is directed specifically to the con-
likely to he economical only in regions of low
trol of pavement distortion caused by frost heave. It
design freezing index.
requires a sufficient thickness of pavement,
a. Air freezing index. Air freezing index values
18-6
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